A composite color video signal includes horizontal synchronizing signals, a burst signal superimposed onto the pedestal level at the back porch of the horizontal synchronizing signal and a video information signal. The video information signal comprises a chrominance subcarrier having different phases amplitude-modulated with chrominance information. The composite color video signal includes both luminance and chrominance information.
Separator circuits are utilized to separate the horizontal synchronizing signal and the burst signal from the incoming composite video signal. The burst signal has a burst signal frequency equal to 3.58 MHz, which is the frequency of the chrominance subcarrier f.sub.SC. When separating the burst signal from the composite video signal it is necessary for the separator circuitry to determine when the burst signal is present on the incoming composite video signal.
In many video transmission systems, color or chrominance information is represented by a particular phase of the chrominance subcarrier signal that is amplitude modulated with color information. Since the phase of the color subcarrier signal is used to represent color information, it is important that, when digitally encoding the color video signal, the phase of the sampling pulses is accurately controlled. Undesired phase shifts, such as may be due to temperature drift, aging of the electrical components, and the like, may result in a phase error in the sampling pulse relative to the chrominance subcarrier signal which has the effect of distorting or interfering with the overall chrominance effect of the video picture which ultimately is reproduced from the digitally encoded video signal.
To identify the aforementioned phase shifts between the sampling pulses and the chrominance subcarrier signal, the instantaneous phase angle of the burst signal at the time of sampling is determined. If the phase angle of the burst signal differs from a desired phase angle, the phase of the sampling pulses may be adjusted accordingly. If the phase angle of the burst signal is different than an expected phase angle, the phase difference between the modulated chrominance information and the signal will also be in error, thereby causing distortion of the color within an output video signal.
It is therefore important to the operation of the video transmission system that the burst signal is separated correctly from the composite video signal in order that the phase of the burst signal can be compared to the phase of a reference signal. Any error in separating the burst signal from the input composite video signal may result in an error determining the phase difference between the two signals and will cause the output video signal to be in error.
Within the composite video signal the values of the components are determined by their relative amplitude with respect to the blank or pedestal level. It is therefore essential that the blank or pedestal level is maintained at a known level so that the value of the component of the composite video signal can be readily determined. In the past, the blank level has been set to a known DC level allowing the values of the components of the composite video signal to be determined by determining their amplitude with respect to the blank level. However, care must be taken when setting the blank level to a specific value that the remainder of the composite video signal is not altered. Video systems of the prior art, in order to set the blank level to a known value, would separate the chrominance and luminance information from the composite video signal and then hard-clamp the blank level of the composite video signal to the appropriate level. Such a system is disadvantageous because at least two additional pins on the integrated circuit and external components are required within the system. What is needed is a system which does not require additional pins and external components and can set the blank level of the composite video signal to an appropriate level without altering the composite video signal.